Glue in the 1870s

The hotter the glue, the more force it will exert in keeping the two parts glued together; therefore, in all large and long joints the glue should be applied immediately after boiling. Glue loses much of its strength by frequent re-melting; that glue, therefore, which has been reboiled. In melting ordinary glue in the double vessel containing water, it is an excellent method to add salt to the water in the outer vessel. It will not boil then, until heated considerable above the ordinary boiling point; the consequence is, the heat is retained, instead of passing off by evaporation, and when the water boils, the glue will be found to be thoroughly and evenly melted.

To prevent glue from cracking. Glue is often found to crack in very dry localities, particularly when the objects glued together are not in close contact, but have a thin layer of glue between them; in which case they sometimes fall apart. Very thin layers of glue are not only exceedingly hard, but also more or less brittle when extremely dry. Therefore to prevent this dry and consequent brittle condition, the addition of a very small quantity of glycerine will accomplish the desired end. The quantity of glycerine must be modified according to circumstances.